(a) Field of the Invention
The present invention relates to a method for an electron beam exposure and, more particularly, to an electron beam exposure method for forming patterns directly on a semiconductor integrated circuit using a moving stage at a constant speed.
(b) Description of the Related Art
An electron beam exposure system generally comprises a continuous stage drive for driving the stage on which semiconductor wafers are mounted at a constant speed.
FIG. 1A shows a semiconductor wafer on which a plurality of chips are arranged for electron beam exposure by using an electron beam exposure system, and FIG. 1B shows the detail of one of the chip surfaces on the semiconductor wafer. In an electron exposure system using a continuous stage drive, the stage mounting thereon the semiconductor wafer 101 is continuously moved in X-direction at a constant speed and the electron beam is also shifted continuously in X-Y directions following the movement of the stage to form patterns on the chip 102.
During the normal patterning on the chip 102, the chip 102 is divided into a plurality of strip regions in Y-direction each corresponding to the width (W) of electron beam deflection, and patterns are consecutively formed in each strip region. The width W of the strip regions depends on the ability of the electron beam exposure unit which determines the dimensions for the electron beam deflection. If the width of the strip regions is set to exceed the deflection capability of the electron beam, pattern degradation will arise due to a large deflection distortion.
During the exposure, semiconductor wafer 101 is moved or shifted in the direction designated by arrow "S" (downward in the drawing). As a result, electron beam is moved relative to the semiconductor wafer in the direction designated by an arrow shown in each strip region (upward in the drawing), while deflected within the strip region in X-Y directions following the movement of the stage to form a desired pattern in each strip region. After a patterning is finished for one strip region, patterning in the next strip region follows by the movement of the stage. In this example, the patterning is conducted in the strip regions c1, c2 and c3 consecutively.
FIG. 2 is a graph for showing a beam deflection width plotted against the stage speed. The stage can be moved at speeds between Vmin and Vmax during the exposure, which depend on the ability of the stage drive. On the other hand, to form a suitable pattern on the chip, a suitable deflection dimension or width of the electron beam resides within an optimum deflection range "a" in FIG. 2 depending on the ability of the beam deflection unit, which is lower than the maximum range "b" corresponding to the maximum speed Vmax and the minimum speed Vmin of the stage. Accordingly, the stage speed is practically limited between V1 and V2 corresponding to the optimum deflection range "a", in order to form desired patterns without deformation.
FIG. 3A shows an example of a chip which has an area "A" having a normal pattern density and an area "B" having a higher pattern density. The normal pattern density area "A" can be patterned by employing the stage speed between V1 and V2 and deflecting the electron beam within the optimum deflection range "a", as shown in FIG. 3B, without deformation. On the other hand, the high pattern density area "B" can be patterned without deformation by reducing the stage speed. However, if the reduced stage speed is below Vmin, the stage cannot be accurately controlled at a constant speed, which causes deformation in the resultant pattern. To avoid the stage speed below Vmin in this case, the deflection width of the electron beam deviates from the optimum deflection range "a", as shown in FIG. 3C, which also causes deformation in the resultant pattern. The low stage speed also reduces throughput of the electron beam exposure.
Patent Publication JP-A-6-151287 proposes an electron beam exposure system wherein patterns are classified into a plurality of groups having different pattern densities. In this case, the electron beam exposure for patterning of a single pattern having a high density is performed in a plurality of times by dividing the pattern into a plurality of sections. In this technique, electron beam deflection does not exceed maximum deflection width in the direction parallel to the movement of the stage. However, this technique cannot also employ an optimum stage speed for each of the pattern densities.